The present invention pertains to a process for the industrial production of dissymmetric secondary amines of general formula R--NH--R', in which R is a fatty chain and R' is a short chain aliphatic residue, more specifically a methyl.
Although industrial procedures are available for the selective preparation of primary amines RNH.sub.2 or dimethylated tertiary amines RN(CH.sub.3).sub.2, no selective process is known for the preparation of monomethylated secondary amines. "Selective" is understood to mean a procedure which produces the monomethylated secondary amine with a minimum amount of the corresponding primary amine and dimethylated tertiary amine.
The specifications for the target derivatives must obviously take into account their intended applications, which include the production of intermediate products for detergents which must have structures which are both oxyalkylated and can be quaternized to a single long aliphatic chain. Although it is possible to use distillation to separate amines with a single fatty chain from amines with two or three fatty chains, there is no convenient method for separating RNH.sub.2 and RN(CH.sub.3).sub.2 amines from secondary RNHCH.sub.3 amines. The boiling points for a given alkyl chain length are very close (approximately 10.degree. C.), and therefore since the general case in this chemistry is to start with raw materials which are cuts with alkyl chain lengths such as C12/C14, C14/C16, C16/C18 or C16/C22, separation of RNH.sub.2, RN(CH.sub.3).sub.2 and RNHCH.sub.3 amines is not attainable. Thus, it is possible to tolerate a certain percentage of dialkyl or trialkyl amines, since they can be separated out by distillation, in proportions such that they do not impact the economy of the procedure. It is also possible to accept without dire consequences a reasonable level of dimethylamines, which follow along with the methylalkylamines as quaternized derivatives. Thus, the presence of monoalkylamines represents the major constraint.
Therefore, the goal comprises the following specifications for commercial dissymmetric secondary amines:
______________________________________ RNHCH.sub.3 min. 85% RNH.sub.2 max. 8% RN(CH.sub.3).sub.2 max. 2% R.sub.2 NH + R.sub.2 NCH.sub.3 max. 6% R.sub.3 N max. 1% Nonamine max. 2% ______________________________________
which requires a procedure which is selective for secondary RNHCH.sub.3 amine.
The prior art contains descriptions of many reactions and procedures for producing monomethylated secondary amines, notably:
(i) French Patent No. 1,595,771 (HENKEL) which describes the implementation of the reaction between a fatty acid and monomethylamine in the presence of a zinc-aluminum catalyst at 310.degree. C., which nevertheless is not sufficiently selective for the monomethylated secondary amine since there is formation of at least 10% of dimethylated tertiary amine;
(ii) European Patent No. EP 34,480 (R.E. GRIGG) in which the use of noble metal hydride complexes of the type RhH(PPh.sub.3).sub.4 makes possible the selective monomethylation, e.g., with methanol, of amines such as butylamine, pyrrolidine or cyclohexylamine without these catalysts being adapted to the industrial production of monomethylated amines;
(iii) French Patents No. 2,370,029, 2,370,030, 2,370,031 and 2,405,921, as well as U.S. Pat. Nos. 4,206,149 and 4,206,150, which describe implementation of the reaction between a fatty alcohol and monomethylamine on copper-based catalysts supported on alumina and containing at least one of the following elements: Cr, Zn, W, Re and Sn. These French patents do not report the selectivity for dimethylated primary and tertiary amines, but it is indicated that noteworthy amounts (&gt;15% by weight) of dialkylated amines are formed. These American patents report reactions catalyzed on Cu-Re/Al.sub.2 O.sub.3 and Cu-W/Al.sub.2 O.sub.3, from which it can seen that it would be difficult to maintain the level of RN(CH.sub.3).sub.2 below 2%; and
(iv) the report to A. BAIKER and J. KIJENSKI (Catal. Rev.-Sci Eng. (27)4, 653-657, 1985) which showed that the reaction: EQU ROH+H.sub.2 N-CH.sub.3 .fwdarw.RHNCH.sub.3 +H.sub.2 O
performed on a fixed bed of Cu/SiO.sub.2 at 220.degree. C. results in at least 3% of RN(CH.sub.3).sub.2. The maximum yield obtain of RNHCH.sub.3 was 75%.